<script type="text/javascript">
    function iframeHeight() {
        var newHeight = $j(window).height();
        var buffer = 180;     // space required for any other elements on the page
        var newIframeHeight = newHeight - buffer;
        $j('iframe').css('height',newIframeHeight);    //this will aply to all iframes on the page, so you may want to make your jquery selector more specific.
    }

    // When DOM ready
    $(function() {
        window.onresize = iframeHeight;
    }
</script>
<iframe id="ifm" frameborder="0" src="<?php echo $portlet_iframe_url; ?>" width="100%" height="100%" style="border-width: 0px;"></iframe>